This invention relates to an image sensor to replace the human eye, and more particularly, to a color image sensor to be used as the eye for a video camera, monitoring camera, robot or the like, capable of obtaining a sharp color image even in indoor light, by improving the color characteristics of a semiconductor solid image pickup device such as a CCD type, an MOS type, a CPD type and a BBD type.
Conventional color image sensors are, generally, color solid image pickup devices having mosaic or stripe shaped color filter arrays positioned and mounted on the photo sensor arrays on the surface of a semiconductor solid image pickup device, being designed to separate the light entering the solid image pickup device picture element part from the color filter into lights corresponding to color elements, and reading out as color signals by the photo-sensor.
However, the spectral transmissivity of these color filters is, for example in the case of a three-color type, as shown in FIG. 1, and the light actually reaching up to the photo-diode in the silicon substrate to be utilized as optical current is only the shaded portion in FIG. 1. That is, in color imaging, the sensitivity is substantially lowered notably as compared with that of a black-and-white image.
Therefore, in order to enhance the optical sensitivity, it is necessary to improve the sensitivity of the silicon photo diode which is a photo-sensor array formed in a silicon substrate, or raise the transfer efficiency or decrease the noise of the transfer unit or drive unit to increase the amplification factor. Furthermore, in order to improve the color balance, it is necessary to adjust the spectral sensitivity characteristic to the visual sensitivity. To improve the transfer efficiency and decrease the noise in the transfer unit or drive unit, however, there has been shortcomings in respect to the existing semiconductor device fabricating process (the fabricating process of solid image pickup device). On the other hand, it has been proposed to reduce the depth of impurity diffusion when a silicon photo diode is produced in order to improve the blue sensitivity which is low in the conventional silicon photo diode, or to laminate a photoconductive film, such as ZnSe-Zn.sub.1-x Cd.sub.x Te (Newvicon film) or an amorphous silicon on a semiconductor substrate as a photo-sensor film, instead of a silicon photo diode.
Therefore, as shown in the characteristics in FIG. 2, the relative sensitivity characteristics of a silicon photo diode, a Newvicon film and an amorphous silicon (a-Si) greatly deviates from the visual sensitivity characteristics of the human eye, and such modifications are not satisfactory as a sensor to replace the human eye. That is, in the prior art, it was a maximum achievement to obtain the spectral sensitivity as shown in FIG. 2, and even these achievements are not satisfactory in that said sensitivity characteristics greatly deviate from the visual sensivitity characteristics. For example, in the Si diode, if formed in a small depth, the blue sensitivity (the region between 400 and 480 nm) necessary to obtain color images in the solid image pickup device was limited at 10 to 20%. That is, since the optical current that can be taken out as an actual color signal was very small in the blue region, the total sensitivity was lowered in the whole region. In the Newvicon film or amorphous silicon film, although the blue sensitivity can be improved, the red sensitivity (above 650 nm) is too intense, or the film forming process is complicated because of the presence of inorganic matter, and it is far from practical to use at the present.